The present invention relates to a turbocharger for an internal combustion engine and, more particularly, to a bearing of a rotary shaft obtained by integrating a turbine blade and a compressor blade.
Generally speaking, a turbocharger for an internal combustion engine is driven by exhaust gas energy. In the turbocharger, a turbine housing and a compressor housing are disposed so as to sandwich a bearing housing of a rotary drive shaft.
A turbine blade which receives pressure and temperature energy of an internal combustion engine exhaust gas and rotates is attached to one end of the rotary shaft, and a compressor blade which is rotated by the power of the turbine blade and compresses air by the centrifugal rotation force to generate boost pressure is attached to the other end of the rotary shaft.
As a radial bearing of the rotary shaft of an exhaust gas turbine (i.e., rotary shaft of the turbocharger), one which is resistant to high-speed rotation and shaft vibration is used. As an optimum radial bearing, a floating metal (or floating bearing) is used.
In a radial floating metal bearing, a proper clearance is set between the inner side of the bearing and the outer side of the rotary shaft and between the outer side of the bearing and the inner side of the bearing housing. Engine oil is applied to the clearance and the rotary shaft is supported in a floating state by an oil film of the engine oil.
Such radial bearings are disclosed in Japanese application patent laid-open publication Nos. Hei 6-42361 and Hei 11-36878.
As described in the publications, a thrust bearing for the turbocharger is often in contact with an end face of the bearing housing on the compressor side relatively apart from the turbine side.
Concretely, by connecting a pipe branched from a lubricant oil path of the body of an internal combustion engine to the bearing housing of the turbocharger for an internal combustion engine, the engine oil is supplied in a manner similar to the bearings of the internal combustion engine.
The engine oil supplied to the bearing housing is force fed into an annular gap formed along the inner side of the radial bearing, an annular gap formed along the outer side, and gaps formed on the right and left side faces of the thrust bearing via oil paths, and an optimum oil film is formed in each of the gaps. With this configuration, a damping action for suppressing shaft vibration which occurs when the rotary shaft rotates at high speed is displayed, and the stable state of the rotary shaft which rotates at 100,000 rpm/min is maintained.
One type of turbocharger uses two radial bearings and another type uses only one radial bearing. In the former type, the radial bearings are disposed on the turbine blade side and the compressor blade side of the rotary shaft. In the latter type, a sleeve-shaped radial bearing which is relatively long in the axial direction is disposed in the center portion of the rotary shaft. There are also two other types; namely a type in which rotation of the radial bearing itself is regulated, and a type in which the radial bearing itself is also rotated in association with the rotation of the rotary shaft.
For both of the radial and thrust bearings of a turbocharger for an internal combustion engine of recent years, a copper alloy material is used. A lead bronze casting typified by CAC603, a sintered material corresponding to the above, and a brass alloy containing about 70% of Cu are the predominant materials.
It is known that, when oil cooling is not considered, the temperature of the engine oil becomes very high, and the bearings are attached by an S component in the engine oil in a high-temperature environment, corrode and turn black. A black corrosion product generated on the surface of a bearing is easily peeled by abrasion, and it causes abnormal abrasion in the bearing.
Particularly, the turbocharger of an exhaust gas turbine type uses exhaust gas heat energy, so that the temperature becomes high. In a high-temperature engine room atmosphere immediately after high-speed and high-load driving of an internal combustion engine, the temperature increases to about 250xc2x0 C. in the radial bearing on the turbine side and to about 180xc2x0 C. in the thrust bearing, so that a high temperature environment occurs in which the black corrosion product is created.
To suppress the black corrosion product generated in the copper alloy materials, it is effective to use a brass alloy material containing about 60% of Cu.
A general brass alloy material has, on one hand, excellent machinability. On the other hand, the machinability severely deteriorates the abrasion resistance. In the case of using the material for a turbocharger bearing, abnormal abrasion occurs in relatively short time due to disturbances such as shaft vibration of the high-speed rotary shaft and metal contamination floating in the engine oil. Under present circumstances, it is difficult to use the material.
An object of the invention is to provide an exhaust turbocharger having excellent durability by a bearing structure with excellent abrasion resistance without generating a black corrosion product even in a high-temperature oil environment.
According to the invention, to achieve the foregoing object, basically, in a turbocharger for an internal combustion engine, a radial bearing for supporting a rotary shaft of the turbocharger is made of a copper alloy containing, as main components, Cu, Zn, Al, Mn, and Si.
In the configuration, by maintaining basic alloy components of a general brass containing Cu and Zn as main components (60% of Zn), a black corrosion product which is generated in the high-temperature engine oil environment can be suppressed. Moreover, by adding Al, Mn, and Si, the hardness of the alloy is increased, and abrasion resistance of the rotary shaft of the turbocharger which rotates at high speed can be also improved.
For example, a turbocharger according to the invention is provided with a radial bearing made of a brass copper alloy containing 54 to 64 wt % of Cu, 0.2 to 3.0 wt % of Si, 0.2 to 7.0 wt % of Mn, 0.5 to 3.5 wt % of Al, and the rest of substantially Zn.
In such a configuration, in a radial bearing for supporting a rotary shaft of the turbocharger, an Mnxe2x80x94Si compound is crystallized as a hard phase in a brass base material. The crystallized compound is an element which improves the abrasion resistance of the rotary shaft.
In order to display abrasion resistance more effectively, the Mnxe2x80x94Si compound is dispersed so as to be elongated in the axial direction of the radial bearing.